Thin-film opals comprising three layers of 440 nm diameter SiO2 spheres were assembled on Pt electrodes and modified with amino groups on the silica surface. Diffusion of anionic, cationic, and neutral redox species through the opals was studied by cyclic voltammetry. The chemically modified opal membranes demonstrate high molecular throughput and, at low pH, selectively block transport of a cationic redox species relative to that of anionic and neutral redox species. This permselective behavior is attributed to the electrostatic interactions that are enhanced by the tortuous pathway within the opal and by the high surface area of the chemically modified spheres.

Diffusion in face-centered cubic (fcc) opals synthesized from 250 nm-diameter silica spheres was investigated by electrochemical methods and finite-element simulations. Opal modified electrodes (OME) ((111) opal surface orientation) were prepared by thermal evaporation of Au onto ∼1 mm-thick opals. Linear sweep voltammetry of Au OMEs in aqueous solutions containing an electroactive molecule and a supporting electrolyte (0.1 M Na 2 SO 4 ) was used to determine molecular diffusion coefficients, D fcc , within the opal. D fcc is related to the diffusion coefficient of the molecule in free solution, D sol , by the relationship D fcc , ) (E/τ)D sol , where E is the interstitial volume fraction of a fcc opal (E ) 0.260 for an infinitely thick opal) and τ is the tortuosity; the tortuosity reflects the increased distance traversed by molecules as they diffuse through the curved interstitial spaces of the opal lattice, and is a function of both the direction of transport relative to the lattice and the number of layers of spheres in the opal lattice. Finite-element simulations are used to compute τ for transport orthogonal to the (111), (110), and (100) surface orientations for 1−7 layers of spheres. Values of τ ) 1.9 ± 0.7 and 3.1 ± 1.2 were obtained from experiment for transport of Ru(NH) 6 3+ and Fe(CN) 6 4-normal to the (111) surface, respectively, in reasonable agreement with a value of ∼3.0 obtained from the simulation.Introduction. The recent interest in opals comprising a closepacked face center cubic (fcc) lattice of spheres (typically SiO 2 or polystyrene of submicrometer radius) is due, in part, to their application in the synthesis of photonic crystals, 1-10 energy storage media, 11-14 novel magnetic materials, 15-17 and sensors. 18 These materials are typically prepared by infusion or diffusional transport of precursor species through the opal lattice, followed by removal of the spheres to create an inverted opal structure. 1,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] Molecular transport occurs within the tortuous interstitial spaces of the opal, Figure 1. Based on geometric factors alone, the effective diffusivity of molecules within the fcc lattice of spheres, D fcc , can be related to the diffusivitiy of molecules in free space, D sol , by eq 1:

The influence of pH and ionic strength on permselective transport in nanoporous opal films prepared from 440 nm silica spheres was investigated by cyclic voltammetry in aqueous and acetonitrile solutions. Three-layer opal films were deposited from a 1.5 wt % colloidal solution of silica spheres onto 25-microm-diameter Pt microdisk electrodes shrouded in glass. The films were chemically modified by immersing them in a dry acetonitrile solution of 3-aminopropyl triethoxysilane. When the surface amino groups of the modified opal films are protonated and there is little or no supporting electrolyte present in solution, the flux of cationic redox species through the opal membrane is blocked because of electrostatic repulsion. The permselectivity is pH-dependent and can be modulated by adjusting the Debye screening length within the nanopores of the opal by changing the ionic strength of the contacting solution.

Formamidopyrimidine DNA glycosylase (Fpg) and endonuclease VIII (Nei) share an overall common three-dimensional structure and primary amino acid sequence in conserved structural motifs but have different substrate specificities, with bacterial Fpg proteins recognizing formamidopyrimidines, 8-oxoguanine (8-oxoG) and its oxidation products guanidinohydantoin (Gh), and spiroiminodihydantoin (Sp) and bacterial Nei proteins recognizing primarily damaged pyrimidines. In addition to bacteria, Fpg has also been found in plants, while Nei is sparsely distributed among the prokaryotes and eukaryotes. Phylogenetic analysis of Fpg and Nei DNA glycosylases demonstrated, with 95% bootstrap support, a clade containing exclusively sequences from plants and fungi. Members of this clade exhibit sequence features closer to bacterial Fpg proteins than to any protein designated as Nei based on biochemical studies. The Candida albicans (Cal) Fpg DNA glycosylase and a previously studied Arabidopsis thaliana (Ath) Fpg DNA glycosylase were expressed, purified and characterized. In oligodeoxynucleotides, the preferred glycosylase substrates for both enzymes were Gh and Sp, the oxidation products of 8-oxoG, with the best substrate being a site of base loss. GC/MS analysis of bases released from γ-irradiated DNA show FapyAde and FapyGua to be excellent substrates as well. Studies carried out with oligodeoxynucleotide substrates demonstrate that both enzymes discriminated against A opposite the base lesion, characteristic of Fpg glycosylases. Single turnover kinetics with oligodeoxynucleotides showed that the plant and fungal glycosylases were most active on Gh and Sp, less active on oxidized pyrimidines and exhibited very little or no activity on 8-oxoG. Surprisingly, the activity of AthFpg1 on an AP site opposite a G was extremely robust with a k obs of over 2500 min −1 .

Endonuclease VIII (Nei), which recognizes and repairs oxidized pyrimidines in the Base Excision Repair (BER) pathway, is sparsely distributed among both the prokaryotes and eukaryotes. Recently, we and others identified three homologs of E. coli endonuclease VIII-like (NEIL) proteins in humans. Here, we report identification of human NEIL homologs in Mimivirus, a giant DNA virus that infects Acanthamoeba. Characterization of the two mimiviral homologs, MvNei1 and MvNei2, showed that they share not only sequence homology but also substrate specificity to the human NEIL proteins, that is, they recognize oxidized pyrimidines in duplex DNA and in bubble substrates and as well show 5′2-deoxyribose-5-phosphate lyase (dRP lyase) activity. However, unlike MvNei1 and the human NEIL proteins, MvNei2 preferentially cleaves oxidized pyrimidines in single stranded DNA forming products with a different end chemistry. Interestingly, opposite base specificity of MvNei1 resembles human NEIL proteins for pyrimidine base damages whereas it resembles E. coli formamidopyrimidine DNA glycosylase (Fpg) for guanidinohydantoin (Gh), an oxidation product of 8-oxoguanine. Finally, a conserved arginine residue in the "zincless finger" motif, previously identified in human NEIL1, is required for the DNA glycosylase activity of MvNeil. Thus, Mimivirus represents the first example of a virus to carry oxidative DNA glycosylases with substrate specificities that resemble human NEIL proteins. Based on the sequence homology to the human NEIL homologs and novel bacterial NEIL homologs identified here, we predict that Mimivirus may have acquired the DNA glycosylases through the host-mediated lateral transfer from either a bacterium or from vertebrates.